1. Field of the Invention
The present invention relates to a device and a measuring method for ascertaining the internal delay time of a CAN bus connection unit or a CAN transceiver.
2. Description of the Related Art
Published German patent application document DE 10 000 305 A1 describes the CAN (Controller Area Network) as well as an extension of the CAN referred to as TTCAN (Time-Triggered CAN). The media access control method used in the CAN is based on bitwise arbitration. In the CAN, the bitwise arbitration is carried out based on a leading identifier within the message to be transmitted via the bus.
As already described in published German patent application document DE 10 2012 200 997, multiple user stations may simultaneously transmit data via the bus system during the bitwise arbitration, without hereby interfering with the data transmission. When transmitting a bit via the bus (transmission signal), the user stations are able to ascertain the logical state (0 or 1) of the bus (reception signal) in parallel. For this purpose, the transmission signal transmitted on the transmission channel is continuously compared with the reception signal. If there is no match at a certain point in time, the sample point, the bus user terminates its transmitting operation, as it must be assumed that a different bus user is attempting to transmit a message having a higher priority or a lower-level identifier.
The reception signal constitutes a superimposition of the message bits of all bus users which are attempting to gain access to the bus during the arbitration. Because of the propagation times of the signals on the bus lines and because of intrinsic delay times in the bus connection units (transceivers), the result of the superimposition of these signals is present only late within the bit period, so that the sample point must be relatively far back within the bit period. Among other things, this fact limits the allowable bit lengths downward in the CAN. A shortening is not readily possible.
Published German patent application document DE 10 2012 200 997 describes how a secondary sample point (SSP) is used to detect bit errors on a CAN bus. In order to determine the position of the SSP, the internal delay time of a CAN bus connection unit or a CAN transceiver DELTA_T may be measured with the aid of a particular method which is described in published German patent application document DE 10 2012 200 997.
The laboratory evaluation of the method described in published German patent application document DE 10 2012 200 997 has demonstrated that in the case of heavily disturbed signals on the CAN bus, measuring errors may occur during the ascertainment of the internal delay time of a CAN bus connection unit or a CAN transceiver.